1. Field of the Invention
The present invention relates to a flip-chip microwave monolithic integrated circuit (MMIC) oscillator or signal source assembly including a resonant inductor in the form of a shorted coplanar waveguide section provided external of the MMIC chip.
2. Description of the Related Art
Radar transceivers incorporating MMIC integrated circuit oscillators and other components are desirable in numerous applications, including near obstacle detection systems (NODS), true ground speed sensors, obstacle avoidance systems (adaptive cruise control), and active phased array radars which utilize a large number of transceivers in a single operating unit. A general treatise on MMIC technology and microwave transmission line configurations is found in "Millimeter-Wave Integrated Circuits", by Kai Chang, TRW Electronics & Defense Sector/Quest, Winter 1983/84, pp. 43-59.
Stringent Federal Communications Regulations of frequency allocation place severe restrictions on the operating bandwidth of radar sensors and microwave communications systems. Conventional MMIC oscillators are generally based on a microstrip architecture, with a relatively large gap between the conductor strips on one side of the substrates and the ground planes on the other side. This gap is conducive to the generation of spurious RF radiation which can exceed the federal standards. In addition, MMIC oscillators often exhibit high phase noise and poor thermal stability because of the relatively low Q-factor of the circuit elements formed on the chips. Frequency locking or complex temperature compensation schemes are often required to achieve acceptable frequency stability or adequate phase noise performance from MMIC oscillators for RF signal generation. An example of such a single chip MMIC oscillator is described in an article entitled "OPTIMIZED X & KU BAND GaAs MMIC VARACTOR TUNED FET OSCILLATORS", by E. Reese et al, in 1988 IEEE MTT-S Digest, pp. 487-490.
An external high Q-factor dielectric resonator is frequently employed to improve the frequency stability characteristics. Replacing the inherently low Q-factor MMIC oscillator resonant circuit elements with an external (off-chip) higher Q-factor dielectric element reduces both the ambient temperature dependance and the phase noise. An example of such an arrangement is found in an article entitled "A BROADBAND VCO USING DIELECTRIC RESONATORS", by P. Kandpal et al, in 1988 IEEE MTT-S Digest, pp. 609-612.
Placement of the external dielectric resonator or "puck" is extremely critical because both the tuning characteristics of the oscillator and the oscillator frequency depend on the coupling and loaded Q-factor of the resonator. Assembly of a dielectric resonator stabilized oscillator requires that the resonator be coupled tightly enough to the MMIC chip by placing it close to the circuit. An external metallic tuner in the form of a screw is employed to adjust the resonator frequency, because it is practically impossible to fabricate many dielectric resonators with identical resonant frequency.
The screw tuning method provides little control of the loaded Q-factor of the oscillator tank circuit, resulting in non-uniform frequency modulation characteristics if the oscillator is employed as a voltage controlled signal source. Mechanical adjustment of the oscillation frequency is labor intensive. Any vibration which changes the gap between the tuning screw and the dielectric puck will produce a fluctuation in oscillation frequency. Despite the high Q-factor of the dielectric resonator arrangement, it is not easily adaptable to low cost, high volume MMIC based microwave module production.